The present invention relates to sliver drafting arrangements used in spinning machines, and in particular to a drafting apparatus having at least one apron drafting zone.
The drafting arrangements for drawing a sliver of parallel fibers are well known in the yarn spinning art. Typical drafting arrangements comprise a series of pairs of rollers of sequentially increasing circumferential roller speeds, the last pair of rollers acting as a pair of sliver delivery rollers to ultimately deliver the drafted sliver to a mechanism for spinning sliver into a yarn, such as a ring spinning or air jet spinning apparatus.
Drafting arrangements utilizing an apron pair, i.e. a pair of apron belts for additional guidance of the drafted sliver are known from, for example, German Auslegeschrift 11 15 160. There, a tensioning device is described for use in conjunction with an upper apron belt of an apron drafting arrangement in a cradle guided with a rigid diverting rail. The bent diverting rail tensions the upper apron by means of a resiliently supporting tension member and is thereby pressed against a fixed stop by an adjusting screw. A nipping area is formed in this apron drafting mechanism between the two aprons. In practice it is difficult with an arrangement in accordance with German Auslegeschrift 11 15 160 to exert a uniform pressure within the apron belts on a sliver to be drawn by the drafting arrangement. More particularly, the pressure between the pair of rollers of the apron drafting arrangement is considered to be greater than the pressure on the cradle or spring loaded arm. As a result, irregular guidance of the fibers in the drafting zone may result which is particularly undesirable for spinning from a twist-free parallel-laid fiber sliver.
As is known in the art, drafting arrangements are typically configured such that the nip lines or areas of each of the pairs of rollers or aprons which comprise the drafting apparatus are arranged in a common plane, in approximately a common plane, or at least in some orientation such that the sliver of fibers being transported through the nip line or area of one set of rollers or apron belts is ultimately delivered into the nip line or area of an immediately following set of rollers or apron belts.
Depending on the exact configuration of the spinning machine, servicing robot, auxiliary mechanisms or other factors involved in a particular spinning apparatus or process, the path of sliver delivery through a drafting apparatus may be arranged such that the sliver emerging from the nip area of a set of rollers or aprons is directed initially along a path of emergence into direct contact with one of a pair of immediately following rollers, whereby the sliver is ultimately routed into the nip of the immediately following rollers.
In one arrangement of this kind (U.S. Pat. No. 4,520,532 Stalder et al.) respective upper and lower guides are provided for the upper and lower apron belts, with the belts being deflected around nose regions of the respective guides adjacent to the upper and lower outlet rollers. This arrangement is particularly contrived so that one of the nose regions projects into the converging space between the inlet and outlet rollers in the direction towards the nip and guides the sliver as closely as possible up to the nip. Moreover, the arrangement is contrived such that, in a beneficial manner, the sliver is deflected over a curved surface of the projecting nose region prior to entry into the nip and also such that the upper and lower apron belts extend closely up to the inlet and outlet rollers. These features are considered beneficial because the deflection of the sliver around the curved nose region of the one apron guide prior to entering into the nip provides additional guidance of the fibers and because the apron belts extending closely up to the inlet and outlet rollers prevent substantial quantities of air being drawn into the nip by the rollers at high speeds of rotation which otherwise causes undesirable disturbance of the fiber sliver. More specifically, if these air streams were not inhibited in some way, they would converge adjacent to the nip and, because of the converging space, would be forced to flow in an axial direction to escape around the sides of the rollers. In so doing they would tend to spread any fiber sliver that may be there and to substantially disturb the orientation of the fibers in the sliver. Despite the beneficial guidance immediately prior to the inlet and outlet rollers provided by the arrangement of U.S. Pat. No. 4,520,532, problems can arise with the guidance of the sliver on its path while sandwiched between the upper and lower apron belts from the upper and lower inlet rollers to the upper and lower outlet rollers. It is noted that this path is essentially a straight path apart from the slight deflection around the nose region of the lower apron guide. Thus, this arrangement, while providing substantial improvements over previous arrangements is still associated with certain guidance problems.
A not dissimilar arrangement is shown in the later U.S. Pat. No. 4,644,609 (Lattner). This reference is not particularly concerned with the guidance of a fiber sliver in a drafting mechanism between two apron belts but rather with the ability to automatically stop and restart the drafting mechanism following strand breakage and repair. This U.S. patent however also shows an arrangement in which the upper and lower apron belts run over upper and lower guide members with the lower guide member being slightly curved and the upper guide member having essentially only a nose region which exerts a guiding function on the upper apron belt as the latter passes around the nose adjacent to the upper outlet roller. The lower guide member with a convexly curved guide surface has a depression or step following the curved guide surface which then merges into a nose portion adjacent the lower outlet roller around which the lower apron belt is deflected. The arrangement is such that neither the upper apron belt nor the lower apron belt is pressed into the depression in the surface of the lower guide member between the convexly curved portion and the nose region. It is essentially only the tension in the upper apron belt which presses the upper apron belt against the curved run of the lower apron belt as it moves around the lower guide member. In practice limits are set on the tension which can be inserted on the upper apron belt, not least because of the need to move the upper apron belt in a sliding movement around the relatively sharply rounded nose region of the upper apron belt guide. Accordingly restrictions are placed on the guiding of the sliver as it passes to the outlet roller pair from the nipping area between the inlet roller pair, and problems can arise with respect to the uniformity of the sliver entering the outlet roller pair and with respect to the uniformity of the subsequently spun yarn. Another arrangement with a lower apron belt guide having a convexly curved surface and an upper apron belt guide defining a nose around which the upper apron belt is deflected immediately in front of the upper outlet roller is disclosed in East German Patent 292,940. If a tangent is drawn to the surface of the lower apron belt immediately before it passes around the front nose region of the lower apron belt guide, i.e. at the point of separation of the sliver from the lower apron belt, then this tangent projects through the nip of the upper and lower outlet rollers and this is indeed the path followed by the fiber sliver. The same is effectively true of the arrangement of the above mentioned U.S. Pat. No. 4,644,609, i.e. the sliver is not subjected to a deflection as it leaves the lower apron belt prior to entry into the nip between the upper and lower outlet rollers. The arrangement of DD 292 940 rather still does not result in a fully satisfactory guidance of the sliver between the apron belts and as it passes into the nip between the upper and lower outlet rollers.
It should also be pointed out that the lack of guidance becomes more critical as the speed of the fiber sliver and the drafting ratio increases.
Finally, for the sake of completeness reference should be made to U.S. Pat. No. 4,533,290 which also discloses an apron belt drafting mechanism rather similar to that disclosed in U.S. Pat. No. 4,520,532 to Kato but with the sliver also moving in a straight line without deflection from the inlet roller pair to the outlet pair while being sandwiched between the apron belts. With such arrangements with the belts moving in a straight line it is difficult to obtain a uniform pressure on the sliver sandwiched therebetween. The special point of the Kato patent is the provision of an apron belt with a long working life.